1. Field of the Invention
The present invention relates to an optical member for splitting or combining luminous flux to be used for observation, examination, or the like, and an observation apparatus provided with the optical member, and an examination apparatus provided with the same.
2. Description of Related Art
Many observation apparatus and examination apparatus, each of which is provided with a light delivery optical system for delivering luminous flux reflected by an object to the eyes of an observer or an imaging (photographing) unit such as a camera, utilize an optical member such as a semi-transparent mirror for splitting or combining the luminous flux.
For instance, in an ophthalmologic field, there has been known a space-saving type of examination apparatus for examining visual functions of an eye of an examinee (see Japanese patent application laid-open publication No. Hei 8-52113 and counterpart U.S. Pat. No. 5,629,748), which is provided with a beam-splitter shaped like a flat-plate disposed between an optotype disc plate and a concave mirror. In this examination apparatus, a light of an optotype is transmitted through the beam splitter and is reflected by the concave mirror toward the beam splitter. The light is then reflected by a half mirror plane of the beam splitter toward the examinee's eye. Such the beam splitter is usually configured of a base body made of a transparent material, such as glass, originally having the internal transmittance of approximately 100% to visible light, in order to reduce the loss of light caused when the light is transmitted through the beam splitter.
Meanwhile, in a configuration that the flat-plate-like beam splitter is obliquely disposed in an optical path of the optical system, a part of the luminous flux split by the half mirror plane is reflected by a back plane of the beam splitter opposite to the half mirror plane, which causes flares and ghost images. Specifically, luminous flux of the optotype which enters the beam splitter is split into main luminous flux which passes through the half mirror plane and first satellite (ghost) luminous flux which is reflected by the back plane. When the main luminous flux is reflected by the concave mirror and returned to the beam splitter, it is split into regular luminous flux which is reflected by the half mirror plane to be directed to the examinee's eye and second satellite (ghost) luminous flux which is transmitted through the half mirror plane. Thus, the examinee can view an image of the optotype, produced by the regular luminous flux. On the other hand, the first and second satellite (ghost) luminous flux which are split from the main luminous flux and the regular luminous flux at the half mirror plane are both reflected by the back plane to form respective optical paths. As a result thereof, the examinee also will view ghost images produced by the satellite luminous flux.
In the optical apparatus provided with the semi-transparent mirror obliquely disposed in the optical path of the light delivery optical system, flares and ghost images are generated as mentioned above. For measures to prevent the generation of the ghost images which may exert an influence on the regular luminous flux, conventionally, the transmittance and the reflectance of the half mirror plane are so designed as to have the most suitable relation therebetween, or the reflectance of an antireflective film applied on the back plane opposite to the half mirror plane is reduced.
However, a manner of designing the transmittance and the reflectance of the half mirror plane with the most suitable relation therebetween has a spec constraint on apparatus configuration. There is also a limitation in the degree of reduction of the reflectance of the film applied on the back plane of the semi-transparent mirror. Accordingly, the above measures are insufficient to reduce the ghost light to an extent that the ghost light does not become a problem.
If the light amount of ghost light is a problem, it is conceivable to lower the light amount itself of luminous flux which enters the semi-transparent mirror. However, the visual function examination apparatus has a fixed minimum light amount of the optotype image, needed for examination, and therefore only lowering the light amount has its limit. In other apparatus, it is naturally desirable to reduce only the light amount of the ghost light while retaining the light amount of the light beam needed for examination or observation.